chore: open v0.3.1-dev cycle on maint

Reset maint to v0.3.0 to retire the v0.2.x maintenance window and
open a new tracking branch for v0.3.1-dev bug-fix work.

- Cargo: 0.3.0 → 0.3.1-dev
- CHANGELOG: fresh [Unreleased] block above [0.3.0]
- README: badge v0.3.0 → v0.3.1--dev

README.md

@@ -3,7 +3,7 @@
 ![banner](docs/logos/fips_banner.png)
 [![License: MIT](https://img.shields.io/badge/license-MIT-blue.svg)](LICENSE)
 [![Rust](https://img.shields.io/badge/rust-1.85%2B-orange.svg)](https://www.rust-lang.org/)
-[![Status](https://img.shields.io/badge/status-v0.3.0-green.svg)](#status--roadmap)
+[![Status](https://img.shields.io/badge/status-v0.3.1--dev-green.svg)](#status--roadmap)
 
 A self-organizing encrypted mesh network built on Nostr identities,
 capable of operating over arbitrary transports without central